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Description

I've recently noticed that App Script is sometimes probing for a property ' __GS_INTERNAL_isProxy' on user created objects/classes. I use a guard Proxy on many of my classes, including public libraries I've created.

I have to update my guard code to treat this as 'not an error' if the property accessed in the Proxy is __GS_INTERNAL_isProxy

This is a breaking change and this new probe will mean updating many scripts.

What are the circumstances under which this property is probed, and are there are any other new breaking changes like this?

Code

  // used to trap access to unknown properties
  guard(target) {
    return new Proxy(target, this.validateProperties);
  },

  /**
   * for validating attempts to access non existent properties
   */
  get validateProperties() {
    return {
      get(target, prop, receiver) {
        // typeof and console use the inspect prop
        if (
          typeof prop !== 'symbol' &&
          prop !== 'inspect' &&
          prop !== '__GS_INTERNAL_isProxy' &&
          !Reflect.has(target, prop)
        )
          throw `guard detected attempt to get non-existent property ${prop}`;

        return Reflect.get(target, prop, receiver);
      },

      set(target, prop, value, receiver) {
        if (!Reflect.has(target, prop))
          throw `guard attempt to set non-existent property ${prop}`;
        return Reflect.set(target, prop, value, receiver);
      },
    };
  },

Output

Steps

• I don't know what prompts apps script to occassionally check this property in custom object. I'm hoping you can tell me.